1. Field of the Invention
The present invention relates to an ultrasonic synthetic aperture diagnostic apparatus capable of detecting the internal information of an object to be examined by successively sending ultrasonic pulse signals into the object by a plurality of transducers, receiving reflected ultrasonic waves reflected by an internal structure of the object by the plurality of transducers every time an ultrasonic pulse signal is sent into the object and detecting the internal information of the object on the basis of the received ultrasonic waves.
2. Description of the Related Art
An ultrasonic diagnostic apparatus has been widely used for diagnosing diseases. The ultrasonic diagnostic apparatus emits ultrasonic waves into a test object, particularly, a human body, receives ultrasonic echoes reflected by an internal structure of the object, converts the received ultrasonic echoes into corresponding input signals and forms an image of the internal structure for diagnosis on the basis of information represented by the input signals.
Referring to FIG. 13 showing a prior art ultrasonic diagnostic apparatus, a probe pulse signal transmitted by a transmitter, not shown, is applied through an amplifier 10 and a probe signal focusing means 12 to transducers 2 included in an ultrasonic probe 1. Then, each transducer 2 emits an ultrasonic pulse signal into an object. Since the delay of the probe pulse signal applied to each transducer 20 is controlled by the probe signal focusing means 12, the ultrasonic pulse signal emitted by each transducer 2 forms an ultrasonic pulse beam, i.e., a probe beam-1 or a probe beam-2, that is focused on a position at a predetermined depth in the object. A delaying pattern in which the probe signal focusing means 12 delays the probe pulse signal is varied to shift the ultrasonic pulse beam in the direction of the arrow shown in FIG. 13 for scanning. The ultrasonic pulse beam is reflected by the internal structure of the object, the ultrasonic echoes reflected by the internal structure of the object are received by the transducers 2, and then the ultrasonic echoes are converted into input signals.
Received signal focusing means 14 delay the input signals provided by the transducers 2 to provide input signals corresponding to the ultrasonic echoes reflected by the same point in the object at the same time simultaneously, and then an adder adds the input signals.
A delay pattern in which the received signal focusing means 14 delay the input signals is varied to execute scanning on the receiving side in synchronism with dynamic focusing in which point on which the input signal is focused is changed sequentially and the scanning action of the ultrasonic pulse beam on the basis of a fact that the ultrasonic wave reflected by a deeper point in the object arrives rater at the transducer 2.
The output signals of the adder 16 are converted into corresponding logarithmic signals by a logarithmic converter 18, the logarithmic signals are detected by a detector 20. Then, the output signals of the detector 20 are converted into image signals representing an image to be displayed on a display 26 by a digital scan converter 22 and the image signals are stored temporarily in a frame memory 24. The image signals are read from the frame memory 24 and sent to the display 26, such as a CRT, to display an image of an internal structure of the object on the display 26 for diagnosis.
With respect to the prior art ultrasonic diagnostic apparatus, several ultrasonic pulse beams which are controlled to focus at several different depth points in the object are needed to make each scanning line. And many scanning lines are needed to make each picture. Therefore#the number of pictures, i.e., the number of frames, which can be formed in unit time is relatively small.
Suppose, for example, that ultrasonic pulse signals are emitted at intervals of 200 .mu.sec, the number of the transducers 2 is 128, the number of scanning lines is 128, a maximum depth is 15 cm, and focusing pitch in the direction of depth is 2 cm. Then, frame rate, i.e. the number of frames per unit times is as small as 4 frames/sec. Therefore, the current ultrasonic diagnostic apparatus sacrifices picture quality for increasing the frame rate.
Efforts have been made to obtain the three-dimensional information of an internal structure of an object by using transducers arranged in a two-dimensional matrix. However, if the number of the transducers is 32.times.32=1024, the number of scanning lines is 64.times.64=4096 and scanning pitch in the direction of depth is 2 cm, frame rate is only 0.14 frames/sec, which is far below a practical level.
An ultrasonic synthetic aperture diagnostic apparatus is one of means proposed to solve such a problem.
FIG. 14 is a diagrammatic view of the previously proposed ultrasonic synthetic aperture diagnostic apparatus and FIG. 15 is a diagrammatic view of assistance in explaining the mode of operation of the ultrasonic synthetic aperture diagnostic apparatus. In FIG. 14, parts like of corresponding to those of the ultrasonic diagnostic apparatus shown in FIG. 13 are denoted by the same reference characters.
Referring to FIGS. 14 and 15, a probe pulse signal transmitted by a transmitter, not shown, is transferred through an amplifier 10 and a switching circuit 28 to transducers 2. Accordingly, the probe pulse signal is applied to one of the M transducers 2 at a time. Suppose that the probe pulse signal is applied to the first transducer 2, i.e., the top transducer 2 in FIG. 14, as shown in FIG. 14. Then, the first transducer 2 emits a ultrasonic pulse signal into the object.
The ultrasonic pulse signal is reflected by the internal structure of the object, represented by a reflecting point in FIG. 14. The M transducers 2 receive the reflected ultrasonic wave and provide input signals. M AD converters 30 sample N input signals corresponding to the reflected ultrasonic waves reflected, respectively, by reflecting points at different depths and converts the N input signals into digital input signals. The digital input signals are stored temporarily in M memories 32. Then, the digital input signals are read from the M memories 32, delayed and added by an input signal focusing means 34 so that R picture elements with respect to the direction of depth in a desired region in the object and Q picture elements (Q scanning lines) with respect to the direction of arrangement of the transducers 2 are formed to form a focused input picture. The focused input picture is given through an adder 36 to and stored in a memory 38.
The switching circuit 28 is set for the second transducer 2, i.e., the second transducer 2 from the top, to emit the ultrasonic pulse signal by the second transducer 2 to make the input signal focusing means 34 provides the next focused input picture by the foregoing procedure. Since the focused input picture obtained in the last ultrasonic scanning cycle is stored in the memory 38, the adder 36 adds the picture elements of the focused input picture previously stored in the memory 38 and the corresponding picture elements of the focused input picture obtained in this ultrasonic scanning cycle, and the focused input picture previously stored in the memory 38 is replaced with a focused input picture obtained by the addition of the picture elements of the successive focused input pictures is stored in the memory 38.
Thus, the M transducers 2 are actuated successively and the focused input picture forming procedure including emission of a ultrasonic pulse signal, reception of the reflected ultrasonic pulse signal and formation of a focused input picture is repeated as shown in FIG. 15 to obtain a display RF data representing the addition of M focused input pictures.
The display RF data is transferred, similarly to the transfer of the input signal in the operation of the ultrasonic diagnostic apparatus, through a logarithmic converter 18, a detector 20 and a digital scan converter 22 to and stored temporarily in a frame memory 24. A tomogram of an internal structure of the object generated on the basis of the display RF data is displayed on a display 26.
The ultrasonic synthetic aperture diagnostic apparatus is capable of operating at a very high frame rate. For example, if the ultrasonic pulse signal is emitted at intervals of 240 .mu.sec, the number M of the transducers 2 is 128, the number of scanning lines is 128 and a maximum depth is 15 cm, the frame rate is 32 frames/sec, which is far higher than the frame rate of 4 frames/sec of the prior art ultrasonic diagnostic apparatus. If the ultrasonic synthetic aperture diagnostic apparatus is provided with 32.times.32=1024 transducers arranged in a two-dimensional matrix for three-dimensional measurement, the frame rate is 4 frames/sec, which is far higher than the frame rate of 0.14 frames/sec of the prior art ultrasonic diagnostic apparatus.
However, since the transducers 2 of the ultrasonic synthetic aperture diagnostic apparatus emit ultrasonic pulse signals at different times, respectively, the ultrasonic synthetic aperture diagnostic apparatus is able to form a clear picture of a stationary object only. For example, in forming a tomogram of an internal structure of an object, the ultrasonic synthetic aperture diagnostic apparatus cannot be satisfactorily focused because the internal structure is caused to move inevitably by the breathing action of the lung and the pulsation of the heart. Accordingly, the tomogram of an internal structure of an object formed by the ultrasonic synthetic aperture diagnostic apparatus is inferior in quality to that formed by the prior art ultrasonic diagnostic apparatus and hence the ultrasonic synthetic aperture diagnostic apparatus has not been applied to practical uses.
Furthermore, since the ultrasonic synthetic aperture diagnostic apparatus synthesizes the display data by calculation after the completion of sequential emission of the ultrasonic pulse signals by the transducers, the ultrasonic synthetic aperture diagnostic apparatus has difficulty in detecting the minute movement of tissues and the velocity of a blood stream by the Doppler analysis.
Still further, it has been impossible to detect a blood stream flowing in a direction perpendicular to the direction of travel of the ultrasonic beam by the Doppler analysis.